The present invention relates to a self-contained energy center which can convert chemical energy into mechanical, electrical and heat energy, and methods for carrying this out.
Such self-contained energy centers or cogeneration systems have been proposed, wherein chemical fuel is combined with compressed air from a compressor and is combusted. The resulting hot, high pressure air is delivered to a turbine which powers the compressor as well as electrical generating equipment. There are thus provided mechanical energy, electrical energy, and heat energy (i.e. waste heat from the turbine) which can be utilized to satisfy various needs such as heating, cooling, ventilating, lighting, etc. in a building.
Such a system is disclosed, for example, in U.S. Pat. No. 4,754,607 wherein fuel and air are combined in a mixer. The resulting mixture is delivered to the inlet of a compressor which compresses the mixture and outputs the compressed mixture to the cold side of a recuperator-type heat exchanger in which it becomes heated. The heated, high-pressure mixture is then delivered to the combustion chamber of a catalytic combustor. The resulting products of combustion are directed to the inlet of an expansion turbine mounted on the compressor shaft. After powering the turbine, the hot combustion gases are directed through the hot side of the heat exchanger. Accordingly, heat from those gases is transferred to the cooler air/fuel mixture passing through the cold side of the heat exchanger. The hot combustion gases exiting the hot side of the heat exchanger are delivered to heat-utilizing devices such as a hot water heater. Meanwhile, the turbine drives an electric generator mounted on the compressor shaft for producing electric power.
During start-up of the system, the combustion chamber of the catalytic combustor is too cold to combust an air/fuel mixture. Therefore, a preheat burner, disposed in the conduit which conducts combustion gases from the turbine to the hot side of the heat exchanger, is supplied with fuel to create combustion gases. Those gases are then supplied to the hot side of the heat exchanger for preheating air delivered to the cold side of the heat exchanger from the compressor (which is being motored-over during start-up). The air preheated in the cold side of the heat exchanger is then conducted through the catalytic combustor to heat the latter. Once the catalytic combustor has been sufficiently heated to support combustion, the start-up phase is over, and fuel is mixed with air fed to the compressor. That air/fuel mixture is then preheated in the heat exchanger before being delivered to the catalytic combustor for combustion therein.
The above-described system exhibits certain shortcomings, especially as regards the start-up operation. In that regard, the start-up procedure requires that heat be transferred from a preheat burner to cold air traveling through a recuperator, and a subsequent transfer of that heat from the air to the catalytic reactor. That procedure is highly time consuming. Also, the need for both a catalytic reactor and a preheat burner complicates the system and increases its cost.
Furthermore, the premixing of air and fuel prior to introduction into the compressor requires the use of a separate mixer, which involves added cost and complexity, as well as a potential source of an energy-diminishing pressure drop which increases the parasitic load on the system, causing the compressor to work harder.
Also, during operation of the system, energy demands will vary, calling for changes in temperature of gas delivered to the turbine. To that end, the amount of fuel supplied to the catalytic combustor can be varied. Also, the system provides a valved by-pass whereby some of the exhaust gases can bypass the hot side of the recuperator, whereby the amount of heat being transmitted to the air/fuel mixture passing through the cold side of the recuperator can be varied. However, a recuperator utilizes a relatively massive heat transfer medium, so there is an appreciable delay before a temperature change at the hot side of the recuperator is realized at the cold side. As a result, the reaction time of the system is slower than would be desired.
Accordingly, it is an object of the invention to provide a highly efficient, low cost, simplified cogeneration system which can be rapidly started.